(82c) Production of water-rich powder by assembling hydrophobic silica nanoparticles

Nowadays, there is a strong need for consumer-friendly products in powder form such as unit-dose products or rapid dissolution formula. They have to be easy to handle and should display a good stability upon storage. It is crucial to master the different stages of particulate processing and to understand the mechanism of particle association at a microscopic level in order to satisfy these demands and optimize the performances of the products. The present work deals with innovative products which look like dry powders but may contain a large amount of liquid water. The solid phase consists of hydrophobized fumed silica, which are agglomerated nanoparticles of silicon dioxide treated with various hydrophobic agents. Unexpectedly, up to 97% (by weight) of water can be incorporated into the solid phase by a simple mixing process. The system obtained is a free flowing fluffy powder. Water can be easily released by mechanical stress, for instance by rubbing it onto the skin. This encapsulation process allows protection and vectorization of water-soluble active agents and can be easily developed at an industrial scale. High level of water encapsulation predicts promising applications in cosmetics, pharmaceuticals and food industry (1). Although this kind of powder was first described in the 60's (2), very few studies have been published so far on the structure of the particulates and on the mechanism of water encapsulation at a microscopic scale (3). The aim of this work was first to study the microstructure of the powder and then to establish correlations between the properties of the system obtained (powder form or not, amount of water entrapped, release of water ...) and the liquid-solid interactions at a microscopic level. Complementary techniques allowing characterization of the powder at different scales were carried out in this purpose. Simple microscopic observation suggested that individual particulates have a characteristic size between 10 and 100 μm. No precise information on the distribution of water inside the solid matrix could be obtained by this technique. A study by Differential Scanning Calorimetry (DSC) of water crystallization in the particulates revealed a behavior similar to water-in-oil emulsions and characteristic of partitioned microsized water droplets (4). This result confirmed that the powder is constituted of small water droplets isolated from their neighbors by fumed silica. In order to reveal the self-association of silica particles around water droplets, a model bidimensional film made of hydrophobic silica particles deposited on the water surface was prepared. Surface pressure-area isotherms showed that the film behaves as an insoluble monolayer in the liquid expanded state. Under high compression, the film collapses and the formation of a gel at the surface of the trough can be easily observed by the naked eye. Gel formation evidences strong interactions between silica particles and the involvement of water molecules. The gel disappeared after decompression of the film. Langmuir-Blodgett film deposition on glass plates and observation through Scanning Electron Microscopy (SEM) showed that silica particles formed a branched network revealing portions of uncovered area. At higher surface pressure, the extent of uncovered area was reduced. From these results, we may expect that individual particulates are made of micrometric water droplets surrounded by a strong silica network which acts like a shell and prevents their collapse. This structure was confirmed through direct observation by freeze-fracture microscopy performed in LVMH Research Center (France) with a methodology previously employed for the observation of multiple emulsions (5). Systematic studies with various fumed silica samples revealed that a determining parameter for successful water encapsulation and powder formation is the hydrophobicity of silica particles. For instance, if the particle hydrophobicity is too low, the energy brought by the mixing process will allow the solid to penetrate the aqueous phase and a concentrated suspension or a foam will be obtained. Indeed, even though treated silica are macroscopically hydrophobic, some hydroxyl groups remain at the surface and may interact with water. The implication of water in the building of the silica network and in the powder formation is still not fully understood. Work in progress includes systematic characterization of treated silica surface hydrophobic character and the correlation with successful powder formation. This project was supported by the Pôle Régional Génie des Procédés (Région Picardie, France).

References : (1) Hydrophobic silica-based water powder, C. Dampeirou, WO Patent 034913 A3, 2005 - Powder to liquid compositions, K. M. Lahanas, US Patent 6290941 B1, 2001. (2) Predominantly aqueous compositions in a fluffy powdery form approximating powdered solids behavior and process for forming same, D. Schutte et al, US Patent 3393155, 1968. (3) Fumed silica for personal care and cosmetics - versatile and effective, S. Hasenzahl, A. Braunagel, SÖFW-Journal, vol 129, August 2003, p.1-8. Liquid absorption capacity of carriers in the food technology, H. Lankes, K. Sommer, B. Weinreich, Powder Technology, vol 134, 2003, p. 201-209. (4) Research techniques utilizing emulsions, D. Clausse, in Encyclopedia of Emulsion Technology (P. Becher Ed.), Marcel Dekker, vol 2, 1985, p. 77-157. (5) Multiple emulsions visualization methods and particle size analysis, L. Robbe-Tomine, C. Le Hen-Ferrenbach, T. Pouget, J-F Tranchant, in Multiple Emulsions : Structure, Properties and Applications (J-L Grossiord and M. Seiller Ed.), Editions de Santé, 1998, p. 141-168.